1. Technical Field
This invention pertains generally to magnetic resonance imaging (MRI), and more particularly to cardiac MRI (CMR) tagging.
2. Background
Cardiac MRI tagging is a promising technique for studying regional heart wall motion noninvasively (both at rest and during stress). In this technique, following the formation of a number of tags in the myocardium through spatially dependent excitation, a sequence of images is acquired at various phases of the cardiac cycle. In principle, many different tagging patterns can be generated through combinations of radio frequency (RF) excitation and gradient pulses. Considering its stability and efficiency, the conventional approach is to generate tagging in the Cartesian coordinate system by the spatial modulation of magnetization, known as SPAMM, in which the short alternating RF and gradient pulses generate a pattern of parallel lines in one or two directions. A similar approach to generate taglines in the presence of RF field inhomogeneities is based on the adiabatic delays alternating with nutations for tailored excitation (DANTE) inversion sequences.
The use of Cartesian tagging was bolstered with the development of the harmonic phase (HARP) concept. This concept considers the uniform and periodic pattern of taglines generated by SPAMM method, which is manifested in spectral peaks in the Fourier domain (k-space) at the fundamental frequency and harmonies of this repetition. The HARP concept helped in automation of the analysis of tagged images which has been a major problem in using tagging techniques for cardiac assessment.
Radial tagging is another pattern of interest that facilitates the measurement of angular information reflected in shear and twist of the left ventricle. Until the recent introduction of an effective pattern and efficient post processing method, this radial modulation of longitudinal magnetization was not being widely used due to the lack of an efficient preparation sequence to generate this tagging pattern robustly.
Yet another tagging pattern of interest is “ring” tagging shown in Spiegel, M. RingTag: ring-shaped tagging for myocardial centerline assessment. Invest Radiol 2003, 38(10):669-78. However, this method selectively excites on-resonance spins over a single curved line (e.g. one singular ring) by shifting the saturation plane off-center (e.g. with an RF pulse having an off-center frequency) and rotating it about an off-plane center point.
Additional background information relating to MRI and CMR can be found in the background art, such as U.S. Pat. Nos. 4,953,554; 5,054,489 5,111,820; 5,217,016; 6,453,187; 6,892,089; and 8,073,523; each of which is incorporated herein by reference in its entirety.